A missing snippet from the manual of http://honk.sigxcpu.org/projects/git-buildpackage/manual-html/gbp.html was how to create a debian native package from scratch, I did something like this:
mkdir newdeb
cd newdeb
git init
git commit --allow-blank -m "Empty upstream, native Debian package"
# or with newer git (thanks marco)
git commit --allow-empty -m "Empty upstream, native Debian package"
git branch upstream
dh_make -p newdeb_1.0 -n -e [email protected] -s
git add debian
git commit -m "Initial blank Debian package template"